This invention relates to voice enhancement and more particularly relates to such enhancement utilizing highly compressed communication signals.
The growth of digital cellular telephones has increased the need for voice enhancement (VE) equipment. A number of products are currently on the market to improve speech quality, including echo cancellers and voice band enhancement (VBE) products, such as acoustic coupling elimination (ACE), noise reduction (NR) and automatic level control (ALC). Products like these are referred to as audio enhancement (AE) products.
When tandem-free operation (TFO) service becomes available, it will no longer be possible to employ these VBE products in the traditional manner. The current known VBE products are designed to process weakly compressed speech data. In TFO, speech is encoded into highly compressed data using various speech compression methods such as those specified in the global system for mobile communications (GSM) standards. The highly compressed speech data are transmitted through the network and are decoded only at the receiver. To apply these products to highly compressed data in the TFO environment requires additional functionality in the network.
To further differentiate between highly compressed data and weakly compressed data, the following definitions are given. Highly compressed data are data whose bit rate is significantly smaller than the bit rate at which it was originally digitized. Such high levels of compression are usually achieved by considering multiple samples of a signal to generate a small number of parameters representing the samples, and involves significant computational expense. Examples of highly compressed data include those compressed using linear predictive coding (LPC) methods, code-excited linear prediction (CELP) methods and multiband excitation (MBE) coding methods. For example, speech data compressed using one of the following standards is considered highly compressed: GSM HR, GSM FR, GSM EFR, GSM AMR and G.728. Thus, highly compressed data includes a range of compression levels (hereafter a “highly compressed range”).
Weakly compressed data are data that include uncompressed digitized audio signals as well as compression schemes that are relatively computationally inexpensive. An example is the G.711 Pulse Code Modulation (PCM) standard. G.711 PCM is a companding scheme used to convert between a 13-bit linear sample and an 8-bit PCM sample. Because of the relationship between the 13-bit and 8-bit samples, the 13-bit samples are also often referred to as linear PCM samples. In the case of the TFO standard, the upper 6 bits of each sample correspond to a PCM code and will be considered as weakly compressed data, while the lower 2 bits correspond to highly compressed data. Thus, the weakly compressed data include a range of compression levels (hereafter a “weakly compressed range”) with less compression than the highly compressed range. Weakly compressed data sometimes are referred to as linear domain signals or data.
The addition of VBE functionality to highly compressed data is a problem which currently confronts the communications industry. Simply adding a decode process on highly compressed data before linear domain (VBE) features and then re-encoding can degrade speech quality. Another approach might be to perform speech enhancements on highly compressed data. However, the enhancement algorithms for the highly compressed data are in their early stages of development and cannot always perform as well as linear domain techniques. The present invention provides the additional functionality needed without the disadvantages of the approaches described above.
One technique for enhancing telephone signals is shown in U.S. Pat. No. 4,283,770 (Stewart, issued Aug. 11, 1981) which describes a processor for multiplying two A-law digitally encoded factors in a manner which produces a product which is a precise linear representation of the product of the linear equivalents of the two factors. Although the Stewart techniques provides some enhancement of telephone signals, it does not teach how to handle TFO service signals for voice enhancement.